The present invention relates to a method and a device for reducing a variable electric field produced by a Cathode Ray Tube (CRT) and equipment controlling it, in its surroundings.
Many electronic devices produce in their surroundings electric and magnetic fields. The inductances, circuit board and wirings of circuits that control a cathode ray tube produce around them an electric field with a variable frequency. The anode of a cathode ray tube also produces around it an electric field with a variable frequency, because the voltage pulses that occur in a deflecting coil connect via capacitance between the deflecting coil and the anode to the anode and further to the surroundings.
Equipment-specific limit values have been defined for electronic devices, such as display terminals equipped with a cathode ray tube, the aim of which is to limit the fields in question. Normally, the limit values are defined as electric field strengths.
FIG. 1 shows, in a simplified manner, the operating principle of a cathode ray tube. At the end of the neck part of the tube, there is an electron gun (egun), which produces an electron beam e. The electron beam is directed onto an image surface at an acceleration voltage which is connected to the tube""s anode A. In addition, the electron beam is modified by control grids G, situated on the tube neck. Around the tube neck part, there are vertical and horizontal deflecting coils Def by means of which the electron beam is directed so that it sweeps the image surface by line from top to bottom. High pulse-like voltages occur in the tube""s deflecting coils and, therefore, the deflecting coils and the conductors connected thereto produce directly around them an electric field and a magnetic field, which is proportional to the voltage in the coils. The voltage pulses that occur in the deflecting coils are also connected to the tube""s anode due to capacitance between the deflecting coil and the tube. The tube""s external surface is coated on the sides with a conductive coating, e.g. graphite, which is connected to a ground potential. In this case, there is also capacitance between the tube""s anode and the ground. A voltage division, capacitive for the tube""s anode, is formed of these capacitances, the capacitances of the voltage division being charged and discharged by pulses occurring in the deflecting coils. In FIG. 1, a capacitor Ca represents the capacitance between the deflecting coil Def and the anode A. A reference Cg represents the capacitance between the tube""s anode and the ground.
The electric fields produced by the windings of a deflecting coil and the conductors connected thereto on the sides and at the back of a display terminal can fairly easily be damped by enclosing the cathode ray tube at the back and on its sides inside a metal or plastic case, which is coated with an electrically conductive coating. In this case, the electric field that is directed straight forwards remains a problem.
A well-known method for reducing an electric field that is directed forwards is to enlarge the capacitance Cg between the tube""s anode and the ground, whereupon the voltage pulses that occur on the tube""s anode become smaller and the electric field produced by the anode becomes smaller. This can be implemented in many different ways, e.g. by coating the tube""s image surface with a transparent electrically conductive film, which is grounded. However, the method is fairly expensive and the film may impair the image quality, and it may become scratched when cleaning the tube.
FIG. 2 shows, at a level of a principle, another well-known method for reducing voltage pulses that occur on an anode. There, pulses Pn, opposite in direction to pulses Pp produced by the deflecting coil of an anode voltage circuit Va, are summed. The summing takes place capacitively via a capacitor Cc. A circuit D supplies deflecting coils Dy. From the same circuit, deflecting pulses are coupled to a compensation coil L, which reverses the pulses in comparison with the pulses, which are coupled to the tube""s anode via the deflecting coils and the capacitance Ca. The compensation coil is dimensioned so that as a result of the summing of the pulses Pn opposite in direction that takes place on the tube""s anode, the sum voltage pulses that occur on the anode are reduced as much as possible. Instead of the tube""s anode, the pulses Pn, opposite in direction, can also be coupled to electrodes on the tube""s sides or to an electrode circulating around the tube.
In compensation methods according to prior art, the basis is that the produced fields have extremely low frequencies and they are produced by the picture tube""s vertical deflecting pulses and they have slightly higher frequencies and they are produced by the picture tube""s horizontal deflecting pulses.
The objective of the invention is to reduce electric fields produced by a display terminal or monitor equipped with a cathode ray tube in its surroundings taking into consideration, in particular, the most common operating situations of the monitor and the electric fields occurring therein.
The present invention flows from the recognition that the picture content presented by the picture tube also has a major impact on the electric field produced by the tube""s anode, whereupon the aforementioned compensation methods according to prior art do not produce the best possible result. The objective of the invention is to develop a compensation method that reduces the electric field produced by a cathode ray tube in its surroundings as effectively as possible by the most commonly used picture content types.
The objectives of the invention are achieved by analysing in a frequency domain the electric field produced by the picture tube""s anode and by examining which components of the frequency spectrum are the strongest and which factors affect different spectrum components. On the basis of the produced result, a compensation method is developed, wherein the frequency spectrum components are weighted in different ways based on the analysis. That what is presented in claim 1 is characteristic to the method according to the invention.
The invention also relates to a compensation device, the amplification and frequency response of which is optimised taking into consideration the effect of the picture content on the electric field produced by the picture tube""s anode. That what is presented in claim 7 is characteristic to the device according to the invention.